Admin notices: error messages should not be all bold

[afercia]

Splitting this out from #47656.

While there are cases where the admin notices error messages may legitimately use bold form some terms within the message, the whole message shouldn't be bold.

The vast majority of the error admin notices are just text with a normal font weight. The <strong> element does add semantic (strong) emphasis but that's an addition that brings in very little vale. Visually: the bold message is inconsistent with most of the error admin notices.

Some examples:

echo '<div class="error"><p><strong>' . __( 'You cannot install a network of sites with your server address.' ) . '</p></div>';

and

echo '<div class="error"><p><strong>' . __( 'The network could not be created.' ) . '</strong></p>';

These "all bold" admin notices should be adjusted to remove the all-bold effect.

Worth also reminding the CSS class class="error" is legacy and should be replaced with class="notice notice-error".

[ocean90]

Related: #37280